This invention relates to techniques for stimulating neural tissue.
The physiology of electrical neuron stimulation is well known. A neuron consists of: a branched pattern of processes, commonly known as dendrites, which act to receive information; a cell body, known as the soma, from which the dendrites extend, and which integrates the received information and provides for the metabolic needs of the neuron; and an axon integral with, i.e., connected to, the soma and extending from the soma, for transporting constituents between the soma and distant synapses, which transfer information to the next set of nerve dendrites.
Neurons may be modelled electrically as distributed capacitors linked by intracellular and extracellular resistances. Neurons are initially, i.e., at a resting time when no stimulation is presented, negatively polarized by about 60 mV-80 mV inside the soma membrane with respect to the outside of the membrane. Depolarization of a soma membrane creates an action potential, which effectively travels via axons to, e.g., the inner brain, thereby sending the stimulation signal to the inner brain. Thus, information is represented in the nervous system as a series of action potentials that travel between the neurons via the membranes of axons. Depolarization of a soma is conventionally achieved by causing current to pass out through a region of the soma membrane. As current flows out of the soma membrane, it acts to largely reduce the polarization potential of the soma, and initiate the soma depolarization, causing an action potential to appear and propagate information along an axon.
In general, outward soma current is typically initiated by transiently forcing the electric potential outside the soma membrane more negative than its resting potential, or conversely by forcing the electric potential inside the soma membrane more positive than its resting potential. In practice, either of these conditions are achieved by flowing current in the appropriate direction through the resistive biological fluids surrounding a neuron.
The outward current flow of depolarization current necessarily causes equivalent inward current flow in another region of the neuron to conserve charge and preserve charge neutrality across the body of the neuron. This independent current largely hyperpolarizes the soma membrane but does not cause an action potential. Neuron stimulation and charge neutrality preservation are thus achieved by manipulation of electrical potentials in the local area of the neuron membrane.
Generally speaking, axons connect neural somas to other somas, as explained above, and thus provide means for delivering electrical information impulses from an exterior neuron to the neurons of the inner brain. Axons pass around many neurons not integral with, i.e., not connected to, those axons in their trajectory from a first neuron to a second neuron at, e.g., the inner brain. As a consequence, external stimulation of a neuron may also unintentionally stimulate an unrelated axon along whose path the neuron is located. Such unintentional axon stimulation results in diffuse, poorly localized neuron activation. This limits the ability to create stimuli indicative of a precise stimuli location in a neural structure.
Referring to FIG. 1, such locally precise stimuli are desirable in, for example, the application of electrical stimuli to retinal ganglia in a visual prosthetic application. FIG. 1 schematically illustrates a region of neural retinal tissue, represented as 2; this tissue includes ganglion somas 4 (tiny dots) and axons 6 (lines). In the case of retinal ganglia stimulation to provide visual signals, stimulus position directly corresponds to visual information encoded in the stimulus. For example, ganglia stimulation with an electrode 6 that, for purposes of discussion, is assumed to ideally focus stimulation, could in theory stimulate a few chosen ganglion somas without stimulating axons around those somas. With this localized stimulation, the brain would then perceive light corresponding to the stimulation to originate at a localized region 8 in the vicinity of the somas. Conversely, ganglia stimulation with an electrode 7 that, for purposes of discussion, is assumed to stimulate a cluster of axons emanating from ganglion somas in a distant region 9 causes an incorrect neural perception of a visual stimulus location at the distant region 9, rather than the intended location at the electrode 7. Stimulation of axons uncorrelated to a neural tissue region of interest is thus seen to provoke unintended, misplaced visual stimulation. Accordingly, the effectiveness of electrical stimulation for retinal prosthetic applications, and indeed any neural stimulation application, is directly impacted by the ability to locally focus neural activations.